Data encoder for power line communications

ABSTRACT

In a disclosed embodiment, a power line communication (PLC) transmitter includes a forward error correction (FEC) encoder that receives payload data and adds parity information to the data to create an encoded output, a fragmenter that receives the encoded output from the FEC encoder and segments the encoded output into a plurality of fragments, a fragment repetition encoder that receives the plurality of fragments from the fragmenter and copies each of the fragments a selected number of times, and an interleaver that receives the copies of the plurality of fragments from the fragment repetition encoder and interleaves the copies of the plurality of fragments for transmission on a power line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 15/796,442 filed on Oct. 27, 2017, which is acontinuation of and claims priority to U.S. patent application Ser. No.15/476,587, filed Mar. 31, 2017 (now U.S. Pat. No. 9,819,392), which isa continuation of and claims priority to U.S. patent application Ser.No. 14/798,576, filed Jul. 14, 2015 (now U.S. Pat. No. 9,667,318), whichis a continuation of and claims priority to U.S. patent application Ser.No. 13/101,938, filed May 5, 2011 (now U.S. Pat. No. 9,112,753), whichin turn claims priority to U.S. Provisional Application No. 61/333,601,which is titled “Header and Method for ITU G-HNEM” and was filed May 11,2010; U.S. Provisional Application No. 61/363,003, which is titled“Header and Method for ITU G-HNEM” and was filed Jul. 9, 2010; U.S.Provisional Application No. 61/443,326, which is titled “Header andMethod for ITU G-HNEM” and was filed Feb. 16, 2011; U.S. ProvisionalApplication No. 61/333,614, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed May 11, 2010; U.S. ProvisionalApplication No. 61/360,559, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Jul. 1, 2010; U.S. ProvisionalApplication No. 61/372,646, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Aug. 11, 2010; U.S. ProvisionalApplication No. 61/383,960, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Sep. 17, 2010; U.S. ProvisionalApplication No. 61/388,863, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Oct. 1, 2010; U.S. ProvisionalApplication No. 61/391,407, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Oct. 8, 2010; U.S. ProvisionalApplication No. 61/392,677, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Oct. 13, 2010; U.S. ProvisionalApplication No. 61/416,809, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Nov. 24, 2010; and U.S. ProvisionalApplication No. 61/418,955, which is titled “Interleaver Design andMethod for ITU G.HNEM” and was filed Dec. 2, 2010, the disclosures ofwhich are hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

Embodiments are directed, in general, to power line communicationsystems and, more specifically, to an interleaver and a header structurefor power line communications.

BACKGROUND

The International Telecommunication Union (ITU) TelecommunicationStandardization Bureau is developing new standards—identified asG.hnem—to enable cost-effective smart grid applications such asdistribution automation, smart meters, smart appliances and advancedrecharging systems for electric vehicles. The G.hnem standards linkelectrical grids and communications networks, enabling utilities toexercise a higher level of monitoring and to support power lines as acommunications medium. The G.hnem standard supports Ethernet, IPv4 andIPv6 protocols, and G.hnem-based networks can be integrated withIP-based networks. The G.hnem standards define the physical layer andthe data link layer for narrowband OFDM power line communications overalternating current and direct current electric power lines atfrequencies below 500 kHz.

The format of the PHY frame and the interleaver that will be used in theG.hnem standards are being considered by the ITU.

SUMMARY OF THE INVENTION

In one embodiment, a transmitter comprises a scrambler circuit forreceiving PHY frames for transmission. The PHY frames comprise a commonheader segment and a data segment. The data segment further comprises anembedded header segment and a payload segment. A header encoder circuitis coupled to the scrambling circuit and provides forward errorcorrection encoding to the common header segment of the PHY frames. Apayload encoder circuit is coupled to the scrambling circuit andprovides forward error correction encoding to the data segment includingthe embedded header segment.

The header encoder circuit may encode the common header segment using amore robust modulation and coding scheme than used by the payloadencoder circuit to encode the data segment including the embeddedheader. The header encoder circuit may encode the common header segmentusing a maximum available Reed-Solomon codeword size. The common headersegment may include data identifying the modulation and coding schemeused to encode the data segment and/or identifying the length of thepayload segment. The embedded header segment and the payload portion maybe encoded using the same modulation and coding scheme. The embeddedheader segment may include a separate cyclic redundancy check.

In another embodiment, a transmitter for a powerline communicationsnetwork comprises a forward error correction (FEC) encoder receivingpayload data and appending parity-check data to the payload data tocreate a FEC bit stream output. An aggregation and fragmentation modulereceives the FEC bit stream and segments the FEC bit stream into aplurality of blocks. An interleaving module interleaves the data in theplurality of blocks.

A fragment repetition encoder is coupled to the aggregation andfragmentation module and receives the plurality of blocks. The fragmentrepetition encoder copies the plurality of blocks a selected number oftimes and provides the plurality of blocks and copies of the pluralityof blocks to the interleaving module. The interleaving module mayinterleave each block and each copy of the blocks separately, or mayinterleave each block and all copies of the block together.

The fragment repetition encoder may combine each block and copies ofeach block in a buffer and pad the buffer with additional bits so thatthe buffer is equivalent to an integer number of symbols fortransmission. The additional bits may be cyclically repeated bits fromthe buffer.

The length of each block may be selected based upon the number ofmodulated bits transmitted in one half-cycle of the power carrier on thepowerline network, or the total number of input bits in a FEC codewordblock, or the total number of bits loaded on symbols that span at least10 ms for a 50 Hz AC powerline communications network, and at least 8.33ms for a 60 Hz AC powerline communications network or for a network withno AC signal, or a maximum fragment size of 3072 bits.

A transmitter in one embodiment comprises a header generation circuitthat provides a set of common header bits and a set of embedded headerbits. A header encoder circuit includes a forward error correctionencoder. The header encoder circuit provides a common header segmentusing the common header bits. A payload encoder circuit includes aforward error correction encoder. The payload encoder circuit provides adata segment that includes an embedded header segment generated from theset of embedded header bits. The data segment may further include a setof embedded payload bits.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, wherein:

FIG. 1 illustrates a G.hnem PHY frame;

FIG. 2 illustrates where padding is used in the signal chain;

FIG. 3 illustrates a functional model of a physical medium attachmentsublayer according to one embodiment;

FIG. 4 is a block diagram of a system in accordance with one embodiment;

FIG. 5 illustrates synchronizing a cyclic prefix with short impulsivenoise bursts;

FIG. 6 illustrates time-domain link adaptation;

FIG. 7 illustrates an interleaver structure according to one embodiment;

FIG. 8 illustrates a functional model of a payload encoder according toone embodiment;

FIG. 9 illustrates the steps performed in the generation of an encodedpayload block;

FIG. 10 illustrates the interleaving process according to oneembodiment;

FIG. 11 illustrates the order of writing and reading into a permutationmatrix according to one embodiment; and

FIG. 12 is an example of row-column permutation that displays thespreading behavior of the interleaver.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Oneskilled in the art may be able to use the various embodiments of theinvention.

FIG. 1 illustrates a G.hnem PHY frame 100, which contains a preamblesegment 101, a header segment 102 and a segment payload 103. Thepreamble 101 enables packet detection. The header 102 enables thereceiver to determine information needed for packet decoding. The header102 can be into two parts: a common header 104 that is transmitted witha fixed modulation and coding scheme, and an embedded header 105 that istransmitted with the same modulation and coding scheme as the payloaddata 103. The embedded header 105 may have a variable length dependingupon the frame type. The embedded header 105 and the payload 103 form adata segment 106 in the PHY frame.

Proposals for the format and contents of the PHY frame header (PFH) tobe used in the ITU-T G.hnem standard are under consideration. Forexample, Tables 1 and 2 list the header fields in two proposals. Theproposals include some initial values for the contents of the G.hnemheader and the number of symbols. Of particular concern is the fact thateven with optimistic assumptions about the header transmission rate(spectral efficiency of 0.5), a large number of header symbols areneeded for the Cenelec bands, resulting in high overhead. The largeoverhead is primarily because of the header size. The common portion ofthe header field totals 42 bits—with additional bits possible fordifferent frame types. Since the header needs to be transmitted with themost robust modulation and coding scheme (MCS), it is desirable tominimize the number of bits transmitted in the header to limit theoverhead.

TABLE 1 NUMBER OF FIELD BITS DESCRIPTION FT 4 Frame type Common part DOD4 Domain ID (identifier) SID 8 The DEVICE_ID of the source node DID 8The DEVICE_ID, MULTICAST_ID or BROADCAST_ID of the destination node(s)MI 1 Multicast indication identifies whether the DID is a unicast ormulticast destination DRI 1 Duration indication- indicates whether theframe duration field is present FTSF TBD Frame-type specific fieldVariable part HCS 16 Header check sequence (16 Common part bits)

TABLE 2 NUMBER OF FIELD BITS DESCRIPTION MPDU Length 7 Indicates thelength of the frame payload expressed in bytes at different scalesdepending on the length of the MAC protocol data unit (MPDU).Transmission 14+ (variable) Indicates the set of parameters used totransmit the Parameters payload. (CM + TM + RS code word size) Ack Req 1Indicates whether the receiver should respond with an acknowledgement toindicate MPDU reception status Op Code 4 Indicates the encoding methodof the remaining bits reserved by ITU in the PFH. Reserved bits forvariable Reserved bits for future use by ITU. upper layer use

TABLE 3 NUMBER OF FIELD BITS DESCRIPTION FT 2 Frame Type Common fieldsHCS 12 Header Check Sequence Sub-total: 14 TM 9-Cenelec Tone MapVariable fields 40-FCC FL 6 Frame Length TX profile 6 ACK 3Acknowledgement FEC size 8 Forward Error Correction size Sub-total:32-Cenelec 63-FCC Reserved 0-Cenelec Variable field  3-FCC Total:46-Cenelec 80-FCC

Embodiments disclosed herein provide methods for more efficient headertransmission, which includes both optimizing the bits contained in theheader, and the modulation scheme for the header.

Header Organization

To minimize the header overhead, the header is split in two parts: acommon header portion (104) and a payload-embedded portion (105). Thecommon part 104 is transmitted with the most robust modulation andcoding scheme (MCS) and the embedded portion (105) is transmitted withthe same MCS as the payload data. The common part 104 signals the MCSused for the payload. The common part 104 also signals the length of thepayload portion. The embedded portion 105 may be embedded with separatecyclic redundancy check (CRC) in the payload. The embedded portion alsosignals the source ID and domain ID.

The contents of the common header 104 in are shown in Table 4 for oneembodiment. Note that it only contains information necessary for thecommon header to receive the data portion: namely the modulation &coding scheme, the bit loading map, and the length of the data.

TABLE 4 NUMBER OF FIELD BITS DESCRIPTION MCS 4 Modulation & codingscheme for data Packet Type 0-3 Indicates packet type LEN 6-8 Length ofthe payload in symbols LEN_EMBHDR 0-3 Length of the embedded header (ifnecessary) BITLOAD 0-6 Bit loading indicator HCS_1 K First K bits of16-bit header checksum, XOR-ed with K-bit destination ID or broadcastgroup ID (Only an intended device passes this portion of the CRC) HCS_216-K Last K bits of 16-bit header checksum (All devices can check thisportion of the CRC)

The following points may be considered in certain embodiments.

The common header (104) may contain an optional length of the embeddedheader, if there is any variability in the embedded header length. Notethat this optional length could be replaced by a packet type, if thereis a unique mapping between the packet type and the embedded headerlength.

The destination ID (or equivalent broadcast group ID) is sent implicitlyby masking the first K bits of the CRC. Only the intended receivers passthis portion of the CRC. Other receivers may check the remaining portionof the CRC to ensure that they have received the length parametercorrectly, and could use this for Carrier Sense Multiple Access (CSMA)or to save power by going to sleep.

The embedded portion (105) of the header contains other fields, such asthose described in Tables 1-4 above, along with a MAC header, if any. Aseparate CRC may be used for the embedded header (105) to enablereceivers to know if they have received the “control” informationcorrectly, even if the actual data is not received correctly.

In another embodiment, the embedded header comprises: a 16-bit sourceidentifier (Source ID); a 16-bit destination identifier (DestinationID); data identifying the number of bits in the payload, which mayexclude padding bits; other segmentation information; a 16-bit CRC; andACK/NAK control. If the embedded header is coded separately from thepayload data, then it may be possible to also add optional power controlinformation in the embedded header to further improve the performance ofdata decoding.

Header Fields

In addition to the organization of the header as discussed above, thechoice of the header fields for various embodiments is discussed below.

MCS:

In one embodiment, the total possible combinations of modulation andcoding scheme (including repetitions) are restricted to twelve or lessfor all subbands. This leaves room for four reserved values, withsixteen combinations of modulation and coding scheme.

LEN:

The length field signals the number of OFDM symbols used. It will beunderstood that the actual signaled value may be a multiple of a basicquantum. In addition, the embedded portion of the header may signal thenumber of padding bits for the payload.

LEN_EMBHDR:

The length of the embedded header may be restricted to a fewcombinations. In one embodiment, the value is limited to fourcombinations.

BITLOAD:

The bit-loading parameter signals the modulation scheme used on alltones. In one embodiment, the available bandwidth may be divided intosubbands. The numbers of tones in the Cenelec A, B, C and D bands are36, 20, 6 and 6, respectively, in one embodiment. On the other hand, forthe FCC bands, the numbers of tones may be, for example, 143, 34, or106. Considering the wide disparity in the number of tones, a differentnumber of BITLOAD bits may be used for different bands. Table 5summarizes the suggested subband size and the number of bits used forsignaling the bit-loading for each band option.

TABLE 5 SUBBAND SIZE No. OF CARRIER (No. TONES/BW IN BITS IN BANDSPACING KHz) BITLOAD Cenelec-A 24.4140625/16 4/6.1035 kHz 3 Cenelec-B 2Cenelec-C 2 Cenelec-D 2 FCC/ARIB 24.4140625/8  4/24.414 kHz 4-5

Length Indication

The main uncertainty in the length comes from padding in the signalchain. This is done in several places, as shown in FIG. 2, which isillustrative of padding used in the signal chain. Padding 0 (201)represents padding of the MPDU that may occur in the data link layer(DLL) before encoding. After passing through a Reed-Solomon (RS) encoderand convolutional encoder, which provide FEC, padding 1 (202) may occurto ensure an integer number of fragments prior to interleaving. Padding2 (203) may occur during interleaving, repetition and concatenation.Finally, padding 3 (204) ensures that a valid frame length is processed.

From the frame duration, the receiver only knows the total number ofbits at the end of the processing chain. It is necessary to define somemethod to ensure the receiver can obtain the other parameters. Twomethods have been proposed to handle this.

In G3, padding 2 and 3 are not performed. The specification does notexplicitly state how padding 0 and 1 are performed, but in oneembodiment padding 0 (201) is done to ensure that the number of bits inpadding 3 (204) is minimized, given the total number of bits to betransmitted. With this assumption, the receiver can uniquely process thereceived bits up till the Reed-Solomon decoder. After that point, DLLsignaling gives the number of bytes added in the padding 0 (201) stage.

This method would work adequately for G.hnem, except for theInterleave-over-AC-cycle (IoAC) mode in which case the padding 2 (203)could add an unpredictable number of bits. To combat this, the followingmethod has been suggested.

Restrict the payload size (i.e. the number of bytes at the input to theRS encoder) to a pre-determined set, and signal the index within thisset. In this method, padding 0 (201) is performed to achieve a targetsize at the RS encoder input, which is then signaled in the PFH. Thisallows the receiver to do the RS decoding, after which DLL signalinggives the number of bytes added in the padding 0 block, just as in G3.This method has the disadvantage of restricting the RS input size, andincreasing the signaling required in PFH.

An alternative method described below builds on both the above methods.

High Level Description of Suggested Signaling Method

In this method, the main idea is to do most of the padding in the DLL(padding 0, 201) as is done in G3. Padding 0 (201) adds an integernumber of bytes before segmentation into LCDUs. The number of bytespadded is indicated in DLL signaling. LCDU segmentation ensures roughlyequal Reed Solomon codeword lengths, subject to a constraint on themaximum LCDU length. By controlling the maximum LCDU length, thetransmitter can adapt to channel conditions. Given the maximum LCDUlength K_(max), the actual codeword length is some number betweenK_(max)/2 and K_(max) depending on the input MPDU length.

Padding 1 (202) adds very few bits—fewer than can be generated by 1 byteat the convolutional encoder input. Thus, the maximum length of padding1 (202) is 15 bits (corresponding to rate-½ code). Since this rule isknown a priori, the number of padding 1 (202) bits is not signaled, butis inferred by the receiver. The values in padding 0 (201) and padding 1(202) ensure that an integer number of interleaver fragments areproduced, while adding the fewest number of pad bits.

Padding 2 (203) is only done for IoAC mode. Note that for eachinterleaver fragment, padding 2 (203) always adds the same number ofOFDM symbols, in order to ensure that each interleaver fragment spansroughly an integer number of OFDM symbols. Note further the number ofOFDM symbols added for each fragment is at most equal 14 for Cenelecbands and 28 for FCC band. Consequently, padding 2 (203) can be signaledwith 4 bits for Cenelec band and 5 bits for FCC band.

Padding 3 (204) is only done to ensure a valid number of OFDM symbols.Assuming this is a multiple of four, then two bits are needed to signalit. Note that padding 3 (204) should not be used in IoAC mode becausethe aim is to spread each frame over an integer number of zerocrossings.

Table 6 lists fields used in one embodiment.

TABLE 6 FIELD NUMBER OF BITS NOTE FT 1 Common HCS 12 fields BAT type 1,indicates run-time Bit Variable Allocation Table (BAT) or not fields TM9-Cenelec 40-FCC If run-time BAT is not used, each bit indicates whetheror not a group is used. If run-time BAT is used, these fields indicateBAT id FL 9-Cenelec 10-FCC In IoF mode, gives the number of symbols inthe payload. In IoAC mode, gives the number of zero crossings, and thenumber of pad symbols used for each fragment during interleaving Coding& 5 (also indicates interleaver type) modulation ACK 1 Maximum RS 2codeword (LCDU) length Reserved 2-Cenelec 8-FCC Total 42-Cenelec 80-FCC

Operation at the Transmitter DLL Padding

The transmitter receives an MPDU size of N_(MPDU-IN) bytes and decideson a maximum LCDU codeword size K_(max), which may be one 4 values inone embodiment (such as 239, 136, 88, and 42, for example). Thetransmitter goes through the following calculations.

First-Cut MPDU Division into LPDUs, and Corresponding Number of CodedBits:

The number of codewords (LCDUs) is N_(CWnom)=ceil(N_(MPDU-IN)/K_(max)).If each RS codeword adds R_(CW) bytes of parity (it is the same for allcodewords, since their sizes are approximate the same), then the totalnumber of bytes at the Reed Solomon output isN_(RSOUTnom)=N_(MPDU-IN)+N_(CWnom)*R_(CW) bytes. This producesN_(FECB)=(N_(RSOUT)*8+6)/r_(I) bits at the convolutional encoder output.

Adjust Number of Reed Solomon Output Bytes:

The number of pad bits (B_(P)) is calculated, namely asB_(P)=B₀×N_(frg)−N_(FECB). These are the number of pad bits that wouldhave been needed, if there were no padding in the DLL. Instead, DLLpadding is done so that 0≤B_(P)<8/rI. More precisely, the desired RSoutput size is N_(RSOUTadj)=floor((B₀×N_(frg)*r_(I)−6)/8). Note thatwith this number of RS bytes, B_(P) is always less than 8/r_(I) asrequired.

Adjust Number of Codewords, MPDU Size:

The actual number of codewords is adjusted toN_(CW)=ceil(N_(RSOUTadj)/(K_(max)+R_(CW))). The adjusted MPDU size isN_(MPDUadj)=N_(RSOUTadj)−N_(CW)*R_(CW). This is the number of inputbytes at the Reed Solomon codeword in order to get an output ofN_(RSOUTadj) bytes.

DLL Padding:

The number of DLL padding bytes is N_(PAD-DLL)=N_(MPDUadj)−N_(MPDU-IN)bytes. These padding bytes are added at the DLL. They could contain allzeros, or maybe cyclic repeats.

Division into Codewords:

The padding input is divided into N_(CW) LCDUs of sizeK_(CW1)=ceil(N_(MPDUadj)/N_(CW)) each, except the last(N_(CW)*K_(CW1)−N_(MPDUadj)) codewords which have size K_(CW1)−1 bytes.

At the end of the above process, the input has been padded to ensure thenumber of padding 1 (202) bits is always less than 8/r_(I), and thepadded MPDU is divided into roughly equal sized LCDUs under theconstraint of the maximum codeword length K_(max).

PITY Padding

The transmitter performs the padding as mentioned above, divides thepadded MPDU into LCDUs, and performs forward error correction (FEC). Thetransmitter than calculates B_(P) padding bits, and does interleaverfragmentation, etc. In Interleave-over-fragment (IoF) mode, no furtherpadding is needed.

In IoAC mode, one further stage of padding is needed. With Rrepetitions, only B₀*R bits are generated by the interleaver. Theoutput, on the other hand, contains ceil(B₀*R/N_(ZC))*N_(ZC) bits. Thus,a difference of Pad2=ceil(B₀*R/N_(ZC))*N_(ZC)−B₀*R bits, or equivalentlyPad3/K_(P) OFDM symbols is added. This number of padded OFDM symbols maybe signaled, given by

N_(pad2-IoAC)=(ceil(B₀*R/N_(ZC))*N_(ZC)−B₀*R)/k_(P) in the FL field forthe IoAC mode.

In addition, to the above padding 3 (204) may be done to get to adesired frame length. If this is done, the value should be signaled.

Summary of Signaling

DLL: Number of pad bytes given by N_(PAD-DLL)=N_(MPDUadj)−N_(MPDU-IN)bytes.

PHY:

1. Maximum codeword size: K_(max). Suggested length=2 bits

2. FL

-   -   a. IoF mode: Number of symbols in frame: 9 bits for Cenelec, 10        bits for FCC band.    -   b. IoAC mode:        -   i. Number of zero crossing cycles: 5 bits for Cenelec, 5            bits for FCC band.        -   ii. Number of symbols in Pad2:            N_(pad2-IoAc)=(ceil(B₀*R/\N_(ZC))*N_(ZC)−B₀*R)/k_(P). 4 bits            for Cenelec, 5 bits for FCC band.

Receiver Operation

IoF Mode:

B_(tot)=k_(P)*FL/R is the total number of bits before repetitions. B₀ isthe maximum divisor of B_(tot) that is less than or equal tomin(B_(tot), N_(ZC), B_(max)).

IoAC Mode:

In IoAC mode, the effective frame length is the total frame length (somemultiple of the number of symbols corresponding to N_(ZC) bits) minusthe number of symbols added in pad 2. (Note that the latter issignaled). With this number, the receiver again calculates:

B_(tot)=k_(P)*FL_(eff)/R as the total number of bits before repetitionand padding by N_(pad2) bits.

B₀ is the maximum divisor of B_(tot) that is less than or equal tomin(B_(tot), N_(ZC), B_(max)).

Once B₀ is known, deinterleaving is done. The number of pad bits in pad1 stage is estimated as B_(P)=mod(B_(tot), 8/r_(I)). These pad bits areremoved before decoding. The DLL contains the number of pad bytes addedin stage 0.

Run-Time BAT Signaling

While run-time BATs (bit allocation tables) are supported in the ITUspecification, there is currently no way for the receiver to determinewhich run-time BAT was used. In order to know this, the receiver has toknow at least (i) the transmitter ID, and (ii) the BAT index.

In one embodiment, a flag may be used to indicate whether the BAT is apre-determined BAT or a runtime BAT. If the pre-determined BAT is used,the tone map field indicates which tone groups are active, as proposedcurrently. If the runtime BAT is used, the tone map field carries thetransmitter (16-bit) ID, the domain (16-bit short) ID and the BAT index.Since these require at least 36 bits, this is done only for FCC band.

For Cenelec band, run-time BAT can only be used with constraints (if atall) since the TM field is not large enough to hold the transmitter anddomain ids. One possible constraint is to restrict run-time BATs inCenelec band so that for a given receiver, the same BAT table is usedfor all transmitters. Thus, just from the BAT index, the receiver candetermine the run-time BAT.

Physical Medium Attachment Sub-Layer (PMA)

FIG. 3 illustrates a functional model of the PMA according to oneembodiment. In the transmit direction, an incoming PHY frame (301)comprises a header (PFH) and payload, such as the PHY frame illustratedin FIG. 1. The PFH may comprise a common part of the header, while thepayload may include a variable or embedded portion of the header. Boththe PFH bits and the payload bits of the incoming frame are scrambled inscrambler 302. The PFH bits of the incoming frame are encoded in PFHencoder 303, which includes a FEC encoder, repetition encoder andchannel interleaver. The payload bits are separately encoded in payloadencoder 304, which also includes a FEC encoder, repetition encoder andchannel interleaver. The PFH and payload encoders 303, 304 may be thesignal chain illustrated in FIG. 2, in one embodiment. The parameters ofpayload encoder 304 are controlled by the PHY management entity 307,which provides PMA_MGMT primitives. The parameters of the PFH encoderare predefined for each particular bandplan to facilitateinteroperability. After encoding, the PFH and payload are each mapped305 into an integer number of symbol frames. The obtained symbol frames(306) of the PFH and the payload are submitted to a physical mediumdependent sub-layer for modulation and transmission over the medium.

In the receive direction, all necessary inverse operations of decoding,and de-scrambling (309) are performed on the received symbol frames(308). The recovered PFH and payload are combined in received PHY frames(310) are further processing in a physical coding sub-layer.

Based on simulations in the Cenelec-A band, it has been agreed to use abit interleaver spanning a half mains cycle. The bit interleaver ispositioned between the FEC and the complex modulation. In extending tothe FCC band, concerns were raised regarding the memory required by aninterleaver with the same time span. On the other hand, for the CenelecB band, it was pointed out that the interleaver may span too few bitsfor the lowest spectral efficiency.

Different interference scenarios may be used for evaluation of thedifferent schemes considered for ITU G.hnem. The noise models to beconsidered for the simulations include white noise only, whitenoise+time domain periodic noise, and white noise+time domain periodicnoise+frequency domain narrowband interference. Two key factors of aninterleaver design are (1) the size of the interleaver, implying thetime period that the interleaver spans, and (2) the complexity ofimplementing the de-interleaver. An interleaver with a longer time spanis expected to spread out impulsive noises. However, an interleaverspanning a longer period in time, implies that the receiver has to dothe de-interleaving operation fast enough to meet any latencyrequirements for acknowledgement of the reception of the packet. Thus,the interleaver length needs to be carefully chosen to achieve atradeoff between performance and complexity. In one embodiment, theinterleaver length is chosen to span the period between the zerocrossings of the AC mains.

Two scenarios are considered, one with a longer interleaver and anotherwith a shorter interleaver whose interleaver length is related to theperiodicity of the time domain periodic noise. The simulation parametersare given in Table 7.

TABLE 7 SIMULATION PARAMETER VALUE Modulation DBPSK Bandwidth occupied40-90 kHz Impulse noise period Various combinations: 11.12 ms, 8.33 ms,10 ms Impulse noise length 2 ms Interleaver size Short & full-packetCoding Rate ½, K = 7 Convolutional code + Reed Solomon

FIG. 4 is a block diagram of a system in accordance with one embodiment.Forward error correction is provided by Reed Solomon coding 401 andconvolutional coding 402. Channel interleaver 403 is 11.12 ms long inone scenario, and 77.84 ms long in another scenario. The output ofchannel interleaver is provided to ODFM modulation 404. The periodicityof the periodic impulse noise is 11.12 ms. Based on observed resultsfrom a set of simulation results for the case when the impulse noiseperiod is 11.12 ms (an integer number of OFDM symbols), the longerinterleaver offers minimal gains at the cost of increased memory andprocessing requirements.

Benefits of Tying Interleaver Length to AC Mains

The results noted above establish that choosing an interleaver having alength that is close to the AC mains suffices to achieve most of theinterleaving gains for the case of periodic impulsive noise. Based onthis, the interleaver length for the Cenelec-A band has been chosen tobe an integer number of OFDM symbols, whose span does not exceed 10 ms.

In one embodiment, the symbol length is also chosen to be equal (or veryclose) to a divisor of the zero crossing duration. In other words, giventhe symbol period T_(sym), the zero crossing period T_(zc) can beexpressed as T_(sym)*N_(sym)+T_(rem), where 0<T_(rem)<T_(sym). In oneembodiment, T_(sym) is chosen so that T_(rem) is close to zero. Thefollowing potential benefits result from selecting Tsym in this manner:

A first minor benefit is that such a choice gives the maximum possibleinterleaver length, under the assumption that the interleaver length isT_(sym)*N_(sym), which is clearly maximized by choosing T_(rem) close tozero. Other choices, with a larger T_(rem), have poorer performance dueto shorter interleaver length, with the degradation being visible aboveT_(rem)˜0.5 ms.

A second benefit of choosing T_(rem)˜0 is that it enables the system tocombat short bursts of periodic impulsive noise. To be precise, considerthe case where the periodic impulsive noise occurs during every zerocrossing, and is less than one cyclic prefix in length (i.e., a few 10 sof microseconds). This case is observed quite frequently, in practice.If there are an integer number of OFDM symbols between successiveimpulsive noise bursts (i.e., between zero crossings of the mains), itis possible to synchronize the cyclic prefix of one symbol with theimpulsive noise burst. Thus, the OFDM system becomes completely immuneto the effect of impulsive noise.

FIG. 5 illustrates synchronizing a cyclic prefix with short impulsivenoise bursts. Note that this is not possible unless there are an(nearly) integer number of OFDM symbols between impulse noise bursts.Even if one impulse noise burst is synchronized with the cyclic prefix,subsequent bursts will drift into the FFT symbol.

Note that achieving the above requires the transmitter to align thestart of some OFDM symbol with the zero crossing. This imposes agranularity of one OFDM symbol length (<1 ms) in the medium accessprocedure. Note that the preamble typically lasts a few OFDM symbols,and so the additional granularity is small compared to the delayrequirements from accurate channel sensing.

Advantages: The above method works without any additional signaling.Further, since the preamble can only start at discrete intervals, thereceiver can use this information to reduce the probability of falsepreamble detections.

Disadvantages: The above method, by itself, is effective only when theimpulsive noise burst length is of the order of the cyclic prefixduration. Further, unless the number of symbols per zero crossing mainsis a multiple of 3, this does not protect against impulsive noise fromall three phases.

For the case of differential PSK, this method provides performancebenefits such that synchronized symbols offer about 0.2-0.3 dB gain overunsynchronized symbols. Further gains are expected with coherentmodulation.

Time-Domain Link Adaptation

To handle the case where impulsive noise is much longer than the cyclicprefix, the system may use time-domain link adaptation as illustrated inFIG. 6. In every block of N symbols 601 a-n, a few symbols are affectedby impulsive noise 602. With simple receiver feedback, the transmittercan adapt the data rate in different symbols 601 to the signal-to-noiseratio (SNR) in the symbol. For example, no data may be transmitted inthe first two symbols 601 a, 602 b, with maximum data rate beingtransmitted in the remaining (N−2) symbols.

Note that the link adaptation mentioned here is the time-domainequivalent of adapting the modulation parameters on different tones inresponse to channel variations or narrowband interference. In powerlinecommunication, since periodic impulsive noise is a dominant noisesource, time-domain link adaptation would be equally effective andshould be combined with frequency domain link adaptation. In the extremecase, the system use a two-dimensional link adaptation for cases wherethe periodic impulsive noise is also frequency selective andconcentrated on a few tones.

The use of time-domain link adaptation only requires receiver feedbackof the phase on which it is present, and the SNR profile acrossdifferent symbols. It does not necessarily require the transmitter tosynchronize symbol start with the zero crossing.

A few different variations are possible, based on the general principleof time domain link adaptation. If some symbols have such high noisethat they cannot be used, then, (i) dummy data can be transmitted, (ii)only pilots can be transmitted, (iii) neither pilots nor data may betransmitted, (iv) neither pilots nor data may be transmitted and thepilot phase may not count up.

In the transmitter, after optional repetition, the coded bit stream isinterleaved 403 before modulation 404 (FIG. 4). To limit decodinglatency, interleaving is done over a block of N_(int) or fewer OFDMsymbols. The values of N_(int) for different bands is given in Table 8.

TABLE 8 FFT SYMBOL DURATION BAND (EXCLUDING CP) N_(INT) Cenelec-A/B/C/D655.36 us 14 FCC 327.68 us 28

The zero crossing duration (AC mains frequency) may be 50 or 60 Hz. Thevalues in Table 8 were chosen to account for the worst case value zerocrossing period of 10 ms. In other embodiments, different profiles maybe defined for different regions, with correspondingly differentinterleaver lengths.

The total number of bits modulated in one OFDM symbol is denotedB_(sym)=Σ_(k)B_(k). With N_(sym) symbols per frame, the coded bit streamcontains N_(sym)*B_(sym) bits, and is divided intoN_(block)=|N_(sym)/N_(int)| blocks, where ┌f┐ denotes the smallestinteger greater than or equal to f. The first N_(block)−1 blocks containN_(int)*B_(sym) bits, corresponding to N_(int) OFDM symbols. Theremaining bits can only fill N_(remaining)=N_(sym)−(N_(block)−1)*N_(int)symbols. To ensure simplicity of implementation and to ensure sufficientinterleaving depth for all bits, repetition bits are padded to the lastblock to extend the length to N_(int-last) symbols. This is done byrepeating the N_(remaining)*B_(sym) bits circularly till the desirednumber of bits is obtained. For a given value of N_(remaining), the lastblock size N_(int-last) is drawn from the set {N_(int), ┌N_(int)/2┐,┌N_(int)/4┐}.

Given a number of symbols N_(remaining), the choice of N_(int-last) fromthe allowed set needs to be determined. One possibility is to adaptivelyset the size depending on noise conditions. Another is to use a fixedmapping from N_(remaining) to N_(int-last). For example, N_(int-last) isthe minimum value in the allowed set which is greater than2*N_(remaining). If no such value exists, N_(int-last) is equal toN_(int).

Thus, note that the ith block contains coded bits modulating a block ofN_(int)(i) successive OFDM symbols, where

$\begin{matrix}{{N_{int}(i)} = N_{int}} & {{{{{for}\mspace{14mu} i} = 1},2,\ldots \mspace{14mu},{N_{block} - 1},{and}}} \\{= N_{{int}\text{-}{last}}} & {{{{for}\mspace{14mu} i} = {N_{block}.}}}\end{matrix}$

In each block, interleaving of bits is done as follows. The input bitsare S/P converted to a B_(sym)×N_(int)(i) matrix by serial-to-parallelconversion, as shown below.

$\; \begin{matrix}\; & \; & {{Bits}(1)} & {{Bits}\left( {B_{sym} + 1} \right)} & \; & \begin{matrix}{{Bits}\left( {\left( {{N_{int}(i)} - 1} \right)*} \right.} \\\left. {B_{sym} + 1} \right)\end{matrix} \\\; & \; & {{Bits}(2)} & {{Bits}\left( {B_{sym} + 2} \right)} & \; & \begin{matrix}{{Bits}\left( {\left( {{N_{int}(i)} - 1} \right)*} \right.} \\\left. {B_{sym} + 2} \right)\end{matrix} \\\; & \; & {{Bits}(3)} & {{Bits}\left( {B_{sym} + 3} \right)} & \ldots & \begin{matrix}{{Bits}\left( {\left( {{N_{int}(i)} - 1} \right)*} \right.} \\\left. {B_{sym} + 3} \right)\end{matrix} \\\begin{matrix}{Bits} \\\begin{matrix}\left( {1,2,\ldots \mspace{14mu},} \right. \\\left. {B_{sym}*{N_{int}(i)}} \right)\end{matrix}\end{matrix} & \rightarrow & \ldots & \ldots & \; & \ldots \\\; & \; & {{Bits}\left( B_{sym} \right)} & {{Bits}\left( {2B_{sym}} \right)} & \; & {{Bits}\left( {{N_{int}(i)}*B_{sym}} \right)}\end{matrix}$

Then, the matrix is interleaved as follows. The (K, L)th element of theoutput matrix is determined as the (k, l)th input element, wherein

k=(α*(K−βL))% B _(sym)

l=(γ*(L−δk))% N _(int)(i), and the integers α, β, γ, δ are chosen toensure

gcd(α,B _(sym))=gcd(δ,B _(sym))=gcd(β,N _(int)(i))=gcd(γ,N _(int)(i))=1.

After interleaving, the blocks are concatenated and the resultantB_(sym)×N_(sym) matrix is used for modulation.

As long as the interleaver size is matched to the periodicity of theimpulse noise, the performance benefit of longer interleavers isminimal. Hence, in one embodiment, the length of the channel interleaver(the interleaver between the channel and the inner code) is limited tothe periodicity of the impulse noise.

Interleaver Structure

FIG. 7 illustrates an interleaver structure according to one embodiment.

The convolutional code 701 receives and processes one or more ReedSolomon code words and produces N_(FEC) bits. These bits need to berepeated appropriately, interleaved and fit into an integer number ofOFDM symbol frames. Each OFDM symbol frame carries k_(H) or k_(P) bitsdepending on whether the symbol frame goes to the header or to thepayload. This is done in the following steps.

The (N_(FEC) bits) input stream is first segmented (702) into B blocksof roughly equal lengths, drawn from a set of valid block lengths. Thelength of each block is determined as the minimum of: (i) the inputnumber of bits N_(FEC), (ii) 2048 (or some other number) bits, and (iii)the number of bits transmitted in one semi-zero crossing interval(either 8.33 ms or 10 ms depending on the region).

Some example combinations of the above-identified parameters are givenin Table 9.

TABLE 9 No. of No. of Average OFDM modulated bits Desired block No. ofno. of bits symbols in in half-zero- length tones per per k_(P)half-zero- crossing period Minimum of OFDM modulated (Column 2 *crossing (Column 4 * (2048, Column N_(FEC) symbols carrier Column 3)period Column 5) 1, Column 6) <1008 36 2 (QPSK) 72 14 (10 ms) 1008N_(FEC) ≥1008 (Cenelec- A) 1008 <2016 36 4 (16-QAM) 72 14 (10 ms) 2016N_(FEC) ≥2016 (Cenelec-A) 2016 <2048 144 2 (QPSK) 288 24 (8.33 ms) 6912N_(FEC) ≥2048 (FCC band) 2048

Each block is repeated R times (703) and applied to R interleavers(704). R is the number of repetitions, and R may be 1, 2, 4 or 8, forexample.

Each interleaver (704) produces B bits. The output of the interleavers(704) is concatenated (705) to yield a buffer of length RB interleavedbits. The output of the buffer is read to yield the desired k_(P)modulated bits for each OFDM symbol. When a buffer is exhausted, thebuffer for the next block is created and used. If the last buffer isexhausted before the frame has been modulated, the last buffer is readagain from the beginning.

Interleaving Depth

For the case of one repetition, the interleaving depth is limited to thehalf-zero crossing period. This is ensured by the condition that themaximum block length is the number of modulated bits in half mains cycleperiod. There are two exceptions to this pattern:

-   -   1) If the number of repeats is larger than 1, the same coded        bits can be spread across multiple 10 ms intervals.    -   2) For the FCC band, the maximum interleaver depth of 2048 bits        may span less than the half mains cycle period. Simulation        results show that this does not excessively affect performance        even when 30% of the band is erased in 20% of the symbols.

Memory Requirement

The interleaver (704) requires memory allotted for only one block at atime. Even if multiple repeats are interleaved, the log-likelihoodratios (LLRs) can be combined in place. In other words, the system onlyneeds to store LLRs at the deinterleaver output. For multiple receivedblocks, the system only has to find the position where a given bit isdeinterleaved, and accumulate the corresponding LLR. Given that themaximum block size is 2048 bits, only 2K bytes of memory is requiredassuming 8-bit LLRs.

Payload Encoder

FIG. 8 illustrates a functional model of a payload encoder 800 accordingto one embodiment. The payload encoder comprises FEC encoder 801, anaggregation and fragmentation block (AF) 802, a fragment repetitionencoder (FRE) 803, and interleaver 804. FRE 803 supports a robustcommunication mode (RCM) and is turned off in case of normal mode ofoperation.

The incoming PHY-frame payload is divided into sequential informationblocks of K bytes per block. Each information block is encoded by FECencoder 801. The bytes in each information block shall be in the sameorder as they are in the corresponding LPDU. The payload bit to betransmitted first is the first in the corresponding information block.

AF block 802 first collects one or more FEC codewords and forms an FECcodeword block. One or more FEC codewords may make up the FEC codewordblock. The FEC codewords may be concatenated into an FEC codeword blockin the order they are output by the FEC encoder and with the same orderof bits. Further, the FEC codeword block is partitioned into fragmentsof roughly the same size—B₀ bits each.

FRE 803 provides repetitions of fragments with the repetition rate of R.If R>1 is set, each fragment shall be copied R−1 times and all copiesconcatenated into the fragment buffer, FB, so that the first bit of eachcopy follows the last bit of previous copy. The total size of the FB isB₀×R bits. FRE 803 supports the values R=1, 2, 4, where the value of R=1corresponds to normal mode of operation. If R=1, an FB contains a singlefragment of B₀ bits.

All fragments of each FB are interleaved. The first fragment of each FBis interleaved by the interleaver I₁ (804). If repetitions are applied,the following copies of fragments in each FB shall be interleaved byinterleavers I₂, I₃, . . . I_(R) (804), respectively. The interleaverI_(r) is obtained by cyclically shifting I_(I) by round(B₀(r−1)/R) bits.

If the number of bits in the fragment B₀ doesn't fit integer number ofsymbols, the fragment shall be padded accordingly prior to theinterleaving. The pad shall be generated by repeating the bits of thefragment starting from the first bit of the fragment, in ascendingorder, until the symbol is filled up.

If the value of B₀ is selected to be less than N_(ZC), the payloadencoder may be set into Interleaving-over-AC-cycle (IoAC) mode or inInterleaving-over-Fragment (IoF) mode. If set to IoAC mode, each FB(containing interleaved fragments and their interleaved copies) shall bepadded to the closest integer number of N_(ZC). The pad shall begenerated by cyclical repeating of the bits of this same FB, startingfrom its first bit. The first bit of the pad shall follow the last bitof the FB and shall be the repetition of the first bit of the same FB.After FB is padded, it's passed for concatenation 805.

If set to IoF mode, the FB shall be passed for concatenation with noadditional bits padded (while the length of the FB is less than N_(ZC)).

The interleaving mode is set based on the number of symbol frames, k, tobe generated (k_(P) for payload symbol frames and k_(H) for headersymbol frames). If k is an integer multiple of the minimum number ofsymbols spanning an AC zero crossing period, the IoAC mode is alwaysused. Otherwise, the IoF mode is used.

The FBs shall be concatenated into an encoded payload block, in theorder of the sourcing fragments. In Normal mode of operation and IoFmode, the encoded payload block is a concatenation of N_(frg) fragmentsgenerated by the AF 802.

The encoded payload block is passed for segmentation into symbol frames.

FIG. 9 illustrates the steps performed in the generation of the encodedpayload block for the case N_(frg)=4. The AF (802) forms FEC codewordblock 901 comprising one or more FEC codewords 902. The FEC codewords902 are concatenated into an FEC codeword block 901 in the order theyare output by the FEC encoder (801) and with the same order of bits.

The value of B₀ shall be calculated as a divisor of the total number ofbits in the FEC codeword block 901. This shall be the maximum divisorwhich value is less than or equal to the minimum of:

-   -   1) The total number of input bits (the FEC codeword block);    -   2) The total number, N_(ZC), of bits loaded in the symbols that        span at least 10 ms for the case of 50 Hz AC lines, and at least        8.33 ms for the case of 60 Hz AC lines or lines with no AC; or    -   3) The maximum fragment size B_(max).

In one embodiment, the value of B_(max)=3072 bits is supported by alltransmitter and receiver nodes. Support for B_(max)=6144 bits may beoptional at both the transmitter and receiver.

In one embodiment, B₀ is rounded up to fit an integer number of OFDMsymbol frames.

The number of fragments is N_(frg)=ceil(s×N_(FEc)/B₀), where s is thenumber of FEC codewords in the FEC codeword block. To obtain integernumber of fragments, the FEC codeword block 901 is padded (903) withN_(frg)*B₀−N_(FEC) zeros.

The FRE (803) provides repetitions of fragments with the repetition rateof R. If R>1 is set, each fragment shall be copied R−1 times and allcopies concatenated into the fragment buffer, FB (904), so that thefirst bit of each copy follows the last bit of previous copy. The totalsize of the FB is B₀×R bits. FRE 803 supports the values R=1, 2, 4,where the value of R=1 corresponds to normal mode of operation. If R=1,an FB contains a single fragment of B₀ bits.

At the output of the payload encoder 800, a bit stream comprisingindividual buffers {FB_(i)} is available containing bits to be mapped tosymbol frames. For any symbol frame, the desired number of bits, asdetermined by the bit allocation table, are read from the bit stream.The first symbol frame starts reading at the first (leftmost) bit of thesymbol stream, and continues reading in the order FB₁, FB₂, etc.

Channel Interleaver

The fragment interleaver interleaves a block of B₀ bits, based on:

-   -   (i) the number of active subcarriers (ASC) per symbol frame; and    -   (ii) the bit allocation table {b_(i)} giving the number of bits        allocated to the i^(th) supported subcarrier.        Both the above quantities could depend on whether the        interleaver fragment corresponds to header and data bits.

Given the above parameters, the fragment interleaver writes bits into atwo-dimensional matrix, permutes them, and reads out the permuted bits.Specific rules are followed for reading and writing.

FIG. 10 illustrates the interleaving process according to oneembodiment.

Step 1 (1001)—Calculation of parameters: The first step is to determinethe number of rows and columns, and the row quantum parameter thatdetermines the rules for reading and writing. The row quantum b iscalculated as the largest integer less than or equal to the numericalaverage of {b_(i)}, i.e., b=floor(mean({b_(i)})).

The number of columns, denoted m, is calculated as the integer closestto (ASC*mean({b_(i)})/b).

The number of rows, denoted n, is B₀/m. The interleaver is defined onlyfor sizes B₀ for which the number of rows is an integer multiple of therow quantum b.

If the bit loading is flat, b equals the number of bits loaded in eachactive carrier, and m equals the number of active subcarriers. In thiscase, n equals the (total number of bits in the interleaver block/m), asrequired by G3. In this case, the number of bits in b rows exactlyequals the total number of bits per symbol frame. If the bit loading isnot flat, note that m does not necessarily represent the number ofactive subcarriers per OFDM symbol frame. Instead, it is chosen toensure that the number of bits in b rows is within b bits of the totalnumber of bits per OFDM symbol frame.

Step 2 (1002)—Writing into permutation matrix: With the above setup,input bits are now written into an n×m matrix as follows. Writing isdone for one quantum of rows at a time. FIG. 11 illustrates the order ofwriting and reading into a permutation matrix according to oneembodiment. Each box in FIG. 11 represents a bit. The number in the boxindicates the position of the bit in the input bit stream. Thus, thefirst b rows are filled before writing into the next b rows, and so on.Within each quantum of rows, bits are written in column-first order. Thefirst bit is written into the first row of the first column, the next iswritten in the second row of the first column, and so on until all brows of the quantum are complete. Then the next bit is written into thefirst row of the second column, and so on.

Step 3 (1003)—Row-column permutation: The entries of the n×m arepermuted. The relation between input and output interleaving indexes isdetermined from the following relations: original bit position (i, j)where i=0, 1, . . . , m−1 and j=0, 1, . . . , m−1.

Interleaved position (I, J) is given by:

J=(j*n_j+i*n_i)% n

I=(i*m_i+J*m_j)% m

where m_i, m_j, n_i, and n_j are selected based the values of m and n,under the constraint that

m_i,m_j,n_i,n_j>2

GCD(m_i,m)=GCD(m_j,m)=GCD(n_i,n)=GCD(n_j,n)=1

FIG. 12 is an example of row-column permutation that displays thespreading behavior of the interleaver for n=8, m=10, n_j=5, n_i=3, m_i=3and m_j=7.

Step 4 (1004)—Reading from permuted matrix: Bits are read from thepermutation matrix in the same order in which they were written.

In one embodiment, two interleaving modes are defined, depending on howthe reading process transitions from one buffer to the next.

In the sub-zero crossing mode, all buffers—except the last one—arediscarded immediately after the last bit in the buffer is read. If thelast bit of the last buffer has been read and there is still a need formore bits to generate symbol frames, reading continues again from thebeginning of the last buffer. In other words, the last buffer is readcircularly until all bits required for symbol frame generation areavailable.

In the zero-crossing mode, a buffer is discarded when the number of bitsread from it is N_(ZC). In this mode, all buffers are read in a circularfashion if necessary.

In one embodiment, the sub-zero crossing mode is always used for the PHYheader and in any case where the number of repetitions R is greater thanone. For the case of one repetition (R=1), the mode may be chosen basedin one of the following ways:

1) One possibility is to choose the mode based on accumulated noisestatistics, which are either fed back from the receiver explicitly orimplicitly determined by the transmitter. In this case, a bit istransmitted in the PHY header to indicate which mode is used for thedata.

2) Alternatively, the zero crossing mode is used only when the number ofOFDM symbols generated is an integer multiple of the minimum number ofOFDM symbols spanning the zero crossing period. In this mode, nosignaling of the mode is done since the receiver can obtain by the modebased on the known value of the number of symbols.

Another alternative is the first symbol frame starts reading at thefirst (leftmost) bit of the symbol stream. Except for the last buffer,every other buffer is discarded after its bits have been read. If thelast bit of the last buffer has been read and there is still a need formore bits to generate symbol frames, reading continues again from thebeginning of the last buffer. In other words, the last buffer iscircularly read till all bits required for symbol frame generation areavailable.

Interleaving Depth for Various Bands

The number of tones in one zero-crossing interval for the various bandsunder consideration is given Table 10 for one embodiment.

TABLE 10 No. OF No. OF No. OF SYMBOLS/ TONES/ TONES/ ZERO ZERO BANDSYMBOL CROSSING CROSSING Cenelec-A 36 14 504 Cenelec-B 17 14 238Cenelec-(C + D) 13 14 182 FCC 107 23 2461

The interleaving depth is limited according to the following rules:

-   -   1) For the Cenelec bands, the maximum memory size of 2K bits is        not relevant. The maximum interleaving depth is 10 ms times the        number of repetitions used.    -   2) For the FCC band, the interleaving depth depends on the        modulation scheme and the number of repetitions. With        (QPSK+repetition-2), note that 2461 bits are contained in 10 ms        and a 2K bit interleaver spans more than 80% of this. Simulation        results show that a 2K bit interleaver handles most realistic        cases without loss of performance.

Handling of Pad Bits and Other Corner Cases

With block interleavers, one corner case that should be handled is thatthe number of bits generated may not fit (i) an integer number of OFDMsymbol frames, or (ii) desired depth in interleaving. Both of thesecases are handled by making the last buffer circular.

Thus, if 300 bits are required for the last symbol frame and only 220are available, then 80 more bits are read from the beginning of the lastsymbol frame. Thus, the first mentioned problem is solved and an integernumber of OFDM symbol frames can always be generated.

Secondly, suppose the FEC output bits only span 34 ms (withrepetitions), and these are divided into four blocks spanning {10, 10,10, 4} ms respectively. The last block only contains 4 ms of data. Ifthe noise conditions are estimated to be benign, the transmitter mayjust transmit 34 ms of OFDM symbol frames, in which case the last blockis only interleaved over 4 ms. Otherwise, the transmitter may transmitOFDM symbols spanning 40 ms in which case the last block is circularlyrepeated as necessary to generate 40 ms. Thus, this structure gives thetransmitter the flexibility to adjust the overhead to the channelconditions without requiring any additional signaling, as describedbelow.

Signaling Required

Note that the receiver can identify the pad bits at all stages ofprocessing, given only (i) the input number of bits N_(FEC), which inturn is uniquely determined by the payload length and code rate, and(ii) the number of OFDM symbol frames. These two quantities need to besignaled anyway, so there is no additional signaling required to achievethe necessary padding.

Memory Requirement

The N_(FEC) input bits are partitioned into one or more blocks. Both theinterleaver and deinterleaver only require memory allotted for one blockof bits (at most 2K bits in length), even if this block is repeatedmultiple times.

Specifically, at the receiver, every time an LLR is generated, thedeinterleaver for the corresponding repeat is calculated and the LLR isaccumulated at the deinterleaver output.

Of course, since processing of one block of bits/LLRs will take time,ping-pong buffers may be required for two successive blocks.

Performance Results

In this section, we provide simulation results to verify performance ofthe proposed interleaver. Table 11 shows the simulation parameters usedfor performance comparison.

TABLE 11 BAND FCC BAND MCS Coherent QP SK with repetition 1 andrepetition 2 Packet size ~2k byte # active tones 150 active tones with3.125 kHz tone spacing Interleaver 10 ms block interleaver, and 2048-bitblock interleaver Channel model Time and frequency erasure channel

In one embodiment, a simulation assumes a periodic noise model wherein afraction of the band gets erased periodically in some fraction of the 10ms duration between zero crossings. This is more realistic than assumingthat the entire 468.75 kHz gets erased during the impulsive noise burst.

It may be questioned whether this is a realistic model instead ofwhole-symbol clipping. The main questions are what is the ADC dynamicrange and whether an impulsive will result in the entire symbol clippingand causing the noise to spread out over the entire band. This dependson

-   -   (i) the energy of the impulsive noise relative to the signal;    -   (ii) the power of the impulsive noise when it is ON;    -   (iii) the automatic gain control (AGC) algorithm used; and    -   (iv) the dynamic range of the analog-to-digital converter.

If the impulsive noise is both high energy and high power (i.e., if itsduty cycle is sufficiently high), the average energy is effectivelydominated by the impulsive noise. Thus, any reasonable AGC algorithmwill backoff so that the impulsive noise does not clip in the ADCsamples. Subsequently, it is straightforward to identify the band inwhich the impulsive noise dominates and erase it. Note that the gains ofdoing so would be significant in cases where the impulsive noise isnarrow in frequency.

Even in cases where the average energy is not dominated by the impulsivenoise energy, the following conditions ensure that entire symbols areoften not erased in the presence of narrowband impulsive noise

-   -   1) With narrowband impulsive noise, the peak-to-average power        ratio (PAPR) is not as high as the desired OFDM signal. Further,        since the impulse noise occurs at every zero crossing, the AGC        could backoff sufficiently at the A/D converter, using an        intelligent AGC algorithm, to ensure that the impulse noise is        not clipped.    -   2) When the impulse noise power is 10-15 dB higher than the        desired signal power, there is not much increased backoff at the        ADC since an OFDM signal needs its own backoff of 10-15 dB at        the ADC anyway. Hence, the increased backoff due to low PAPR        impulse noise is not significant.    -   3) Even if a narrow burst of impulsive noise is clipping at say        12 dB above the signal's rms value, the effective noise is        frequency domain is small enough (its energy is divided by the        ratio of the impulsive noise burst duration to the OFDM symbol        duration) that the entire symbol is not completely erased.

Consequently, we recommend that decisions not be based purely on erasuremodels, in particular for the FCC band which is broad enough to ensuremany impulsive noise bursts affect only a fraction of the tones.

Under the assumptions listed above, and with coherent QPSK withrepetition 2 in 20% time erasure and 20% frequency erasure channel, theperformance difference between the 2048-bit interleaver and the 10 msinterleaver at 1e-2 FER is only 0.1 dB.

Under the assumptions listed above, and with coherent QPSK withrepetition 2 in 20% time erasure and 50% frequency erasure channel,severe portions of the frequency bands are erased. In this channel,there is a 0.6 dB performance difference between two interleavers.

Under the assumptions listed above, and with coherent QPSK in 20% timeerasure and 20% frequency erasure channel, the performance differencebetween 10 ms interleaver and the proposed interleaver is 0.8 dB.

Additional points to consider include:

-   -   1) If it turns out that indeed multiple symbols are wholly        erased, the repetition ratio used would increase. This, in turn,        makes the effective interleaver depth wider. Specifically, with        8 repetitions, the interleaver depth discussed here        (corresponding to 2048*8 bits) is almost equal to 10 ms anyway.    -   2) In the above case, note that much greater gains can be        obtained by using time-domain link adaptation.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions,and the associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

What is claimed is:
 1. An electronic device comprising: a forward errorcorrection (FEC) encoder configured to receive payload data and addsparity information to the data to create an encoded output; a fragmenterthat receives the encoded output from the FEC encoder and segments theencoded output into a plurality of fragments; a fragment repetitionencoder that receives the plurality of fragments from the fragmenter andcopies each of the fragments a selected number of times; and aninterleaver that receives the copies of the plurality of fragments fromthe fragment repetition encoder and interleaves the copies of theplurality of fragments for transmission on a power line.